1. Field of the Invention
This invention relates to a process for the production of cross-linked resilient automotive body components having a cured surface coating. More particularly, this invention is concerned with a process for the production of resilient automotive body components suitable for use as automotive fascia, fender extensions, grilles and the like from an injection-moldable or extrudable thermoplastic elastomeric composition which is free from added vulcanizing agents such as reactive sulfur compounds, quinones, halogenated compounds and reactive resins or free-radical inducing agents such as peroxides and azo compounds.
Most particularly, this invention relates to a process for the production of automotive body components which comprises: fabricating an uncured component by injection molding, thermoforming, stamping from a formed sheet or fabrication processes well known in the art, a composition comprising a uniform mixture of one or more interpolymers comprising ethylene and propylene, one or more homopolymers of a C.sub.2 to C.sub.6 alpha-olefin, one or more multifunctional vinylic or allylic monomers, and optionally a quantity of one or more medium to high structure forms of carbon black sufficient to both reinforce the component and to render the fabricated component electrically conductive; applying a radiation-curable decorative surface coating to the uncured component, preferably by electrostatic means; and simultaneously curing the surface coating and cross-linking the composition comprising the fabricated component by exposing the coated component to high-energy ionizing radiation.
Recently, the need for improved fuel economy in the operation of automobiles has led to smaller cars and automotive manufacturers lowering the weight of automobiles by substituting steel, particularly in the body, with light metal alloys and polymeric compositions. Currently, automotive companies are developing elastomeric structures (fascia) for the front and rear ends of automobiles to replace the present steel fender extensions, radiator grilles and the like. These structures are required to flex on impact in concert with energy absorbing devices and return, undamaged, to their original shape when the distorting forces are released. In addition, the structure must readily accept paint and the finished surface must have adequate weather resistance and show a minimum of marking or marring on impact and recovery.
Suitable and commercially acceptable resilient body components must not only be capable of being amenable to mass production methods, as by conventional injection-molding techniques, but the finished product must possess the necessary physical properties of high flexural stiffness, high tensile strength, hardness, ability to recover rapidly to its original state when deformed and to be mar and tear resistant. In addition, the fascia structures can also be finished by applying a decorative surface coating by electrostatic means; there said structures should be electrically conductive and must not undergo deformation during the paint curing operation.
2. Prior Art
Currently, two principal types of polymeric compositions being used for the fabrication of resilient body components are exemplified first by U.S. Pat. No. 3,915,928 in which the composition comprises an injection moldable mixture of a crystalline copolymer of ethylene and propylene or a terpolymer of ethylene, propylene and a non-conjugated diolefin; carbon; from about 5 to about 30 weight percent of chopped glass fibers; and sulfur-based vulcanizing agents. The presence of the glass fibers in the composition may result in an abrasion of the mold surfaces and parts molded from this composition show "trail" lines due to alignment of the glass fibers at the surface and the parts must be routinely sanded and buffed prior to painting in order to produce acceptable automotive fascia.
In the other, in which the end product is a microcellular polyurethane, described in a paper by Prepelka and Wharton "Reaction Injection Molding in the Automotive Industry," Journal of Cellular Plastics, p. 87, March/April 1975, the cost of the components comprising the composition is higher than the cost of hydrocarbon based elastomers and production of the polyurethane structures requires specialized metering and mixing equipment and presses.
Most recently, resilient automotive body components have been produced from a composition and by a process disclosed in U.S. patent application Ser. No. 864,340 filed Dec. 27, 1977, which is hereby incorporated by reference, in which a first mixture of crystalline interpolymers comprised of ethylene and propylene, a low-density polyethylene and carbon is uniformly mixed to form a second mixture with one or more vinylic or allylic monomers and a particular type of an organic peroxide at a temperature below 130.degree. C. and said second mixture is injection molded and cross-linked in the mold at elevated temperatures.
Cross-linking of thermoplastic and elastomeric polymers by exposure to high-energy ionizing radiation is old in the art. A sampling of recent U.S. Patents and literature which illustrate the art are: U.S. Pat. Nos. 3,911,202, 3,988,227, 3,990,479, and articles in Modern Plastics, p. 55 (1974); T. G. Mysiewicz; Plastics Technology, p. 51 (1977); Business Week, p. 38B, July 11, 1977; Morganstern & Becker, "The Technology and Economics of Radiation Curing," Rubber Division, A.C.S. Cleveland, Ohio, May 1975; R. F. Grossman "Compounding For Radiation Cross-Linking," 1st International Meeting on Radiation Processing, Puerto Rico, May 1976; Smidle, "Radiation," Rubber & Plastics News 13, Sept. 19, 1977; "Radiation Processing Branches Out," Modern Plastics, October 1976; and Bohm et al, "Comparison of Radiation and Sulfur Cured Elastomers," Radiation Dynamics, Inc. Publication 1976.
Curing decorative coatings applied to vulcanized substrates by exposure to high-energy ionizing radiation is also old in the art. A sampling of patents and literature which illustrate the art are: U.S. Pat. Nos. 3,773,638, 3,809,569; 3,560,245; British Pat. Nos. 1,182,079; 1,264,579 and articles by S. E. Young, "The Curing of Organic Coatings By High Energy Radiation Processes," in Progress in Organic Coatings 4, 225-49 (1976) and J. C. Mileo, "Coating Compositions Crosslinkable by Electronic Bombardment," Rev. Inst. Franc. Petrole 31, 665-85 (1976).